(1) Field of the Invention
The present invention relates to optical transmission systems, and more particularly, to an optical transmission system that performs WDM (Wavelength Division Multiplex) optical transmission.
(2) Description of the Related Art
Optical communication network techniques form the core of multimedia communication, and there is an increasing demand for better service that covers a wider area. As the number of Internet users has drastically increased in recent years, the transmission capacity has also sharply increased. In a conventional transmission system that transmits an optical signal of one wavelength through a single fiber-optic cable, the transmission capacity is not large enough. With such an insufficient transmission capacity, data transmission involving video data takes a user a long time.
As a technique of utilizing already available fiber-optic cables, a WDM technique has been developed. In WDM transmission, lights of different wavelengths are multiplexed, and signals of a plurality of channels are simultaneously transmitted through a single fiber-optic cable.
FIG. 20 shows the structure of a WDM ring network. Nodes 301 through 304 that perform WDM transmission are connected to one another with a fiber-optic cable, and thus constitute a ring structure. A plurality of SONET/SDH transmission devices 301a through 301d are connected to the nodes 301 through 304, respectively, and perform OADM (Optical Add Drop Multiplex) control.
The nodes 301 through 304 each multiplexes a signal “Added” by each corresponding one of the transmission devices 301a through 301d, so as to transmit the signals through the single fiber-optic cable within the ring. Each of the nodes 301 through 304 also performs photodisintegration on signals transmitted within the ring, and then “Drops” the signals onto the transmission devices 301a through 301d. 
FIG. 21 shows the structure of the nodes that perform OADM control. The node 301 includes an optical switch 301-1 and an AWG (Arrayed wave-guide Grating) device 301-2. The node 302 includes an optical switch 302-1 and an AWG device 302-2.
The optical switch 301-1 of the node 301 performs a switching operation on an optical signal transmitted within the ring and an optical signal to be “Added” or “Dropped” to the transmission device 301a. The AWG device 301-2 performs a combining operation on optical signals outputted from the optical switch 301-1 so as to generate a wavelength-multiplexed signal. The AWG device 301-2 then outputs the wavelength-multiplexed signal onto the fiber-optic cable.
The AWG 302-2 of the node 302 performs a branching operation on a received wavelength-multiplexed signal. The optical switch 302-1 performs a switching operation on a branched optical signal and an optical signal to be “Added” or “Dropped” to the transmission device 302a. 
In the above manner, the WDM network performs OADM control on each node, and generates and transmits a wavelength-multiplexed signal with the AWG device that performs optical wavelength combining and branching operations. The WDM network thus performs optical communication.
The above described AWG devices are essential in construction of a WDM network. A conventional AWG device is formed by an optical circuit employing optical wave-guides of quartz glass (that are suitable for mass production, and therefore are often employed in WDM systems). Such a conventional AWG device divides optical signals of different wavelengths by the wavelength.
AWG devices can be classified into two types: one is a Gaussian type exhibiting a Gaussian waveform as the transmission characteristics (or the loss characteristics), and the other is a flat top type exhibiting a flat waveform. In general, flat top type AWG devices are employed in high-speed and large-capacity WDM networks.
However, each flat top type AWG does not have a completely flat optical spectrum, and causes small distortions in practice. Accordingly, distortions accumulate in an optical signal that has passed through a plurality of flat top type AWG devices via a plurality of nodes.
As a result, the flatness is lost at the center wavelength and in the vicinity of the center wavelength of the unit wavelength, and the transmission quality degrades. Accordingly, in each conventional WDM network employing flat top type AWG devices, the number of nodes to be provided is limited, so as to prevent transmission quality degradation (i.e., so as not to lose the flatness of the optical spectrum) Because of the limited number of nodes to be provided, an optical network having a high operation efficiency could not be constructed.
Meanwhile, the flatness of each AWG device can be improved by employing a large number of optical filters. However, this leads to a large loss, and requires an optical amplifier for compensating such a loss. Accordingly, employing a large number of optical filters results in higher production costs, as well as optical S/N degradation due to the accumulation of optical noise.
Furthermore, in a case where the flatness is measured by a spectrum analyzer on the reception end so as to perform a feedback control operation, it is necessary to prepare a wideband test light source, such as a white light source, and a spectrum analyzer on the transmission side.